1. Technical Field of the Invention
The present invention relates generally to steering apparatuses or systems for vehicles. More particularly, the invention relates to a steering apparatus for a vehicle which is configured to provide suitable steering assistance to the driver of the vehicle so as to improve the turning stability of the vehicle.
2. Description of the Related Art
A conventional steering apparatus for a vehicle is configured to provide a correction steering angle to a steerable road wheel of the vehicle. Specifically, the steering apparatus estimates a cornering force and a slip angle of the steerable road wheel based on the running state of the vehicle and a steering input from the driver of the vehicle. Then, the steering apparatus calculates the partial differential of the cornering force with respect to the slip angle. When the calculated partial differential has a negative value, the steering apparatus provides the correction steering angle, which has a value equal to the difference between the slip angle and a predetermined limit slip angle, to the steerable road wheel.
Such a steering apparatus has recently been developed to include therein a Variable Gear Ratio Steering (VGRS) mechanism that works to vary the ratio between a steering angle of the steerable road wheel and a turning angle of a steering wheel of the vehicle. For example, when the vehicle is running at high speed, the ratio may be made small so as to suppress rapid increase of the steering angle of the steerable road wheel with respect to the turning angle of the steering wheel, thereby securing the turning stability of the vehicle. On the contrary, when the vehicle is running at low speed, the ratio may be made large so as to allow maneuvers of the vehicle that require a large steering angle of the steerable road wheel, such as a parallel parking, to be completed with a small turning angle of the steering wheel.
More specifically, when the turning angle of the steering wheel is made excessively large, the slip angle of the steerable road wheel will accordingly be excessively large. Consequently, the partial differential of the cornering force of the steerable road wheel with respect to the slip angle will have a negative value, in other words, the lateral force of the steerable road wheel will decrease. In such a case, the steering apparatus provides the correction steering angle, which is equal to the difference between the slip angle and the predetermined limit slip angle, to the steerable road wheel. As a result, the actual steering angle of the steerable road wheel is decreased, in other words, the ratio between the steering angle of the steerable road wheel and the turning angle of the steering wheel is decreased, thereby preventing a decrease in the lateral force of the steerable road wheel and securing the turning stability of the vehicle.
However, in the above steering apparatus, since both the cornering force and the slip angle of the steerable road wheel are obtained by estimation, the value of the partial differential of the cornering force with respect to the slip angle will be easily influenced by errors in sensing operating parameters of the vehicle and the turning angle of the steering wheel. Consequently, it is difficult to accurately determine whether the lateral force of the steerable road wheel is to decrease, and thus it is impossible to provide a suitable correction steering angle to the steerable road wheel.
A conventional steering apparatus of another type is configured to detect an actual yaw rate of the vehicle, determine a required yaw rate of the vehicle based on operating parameters of the vehicle, determine a correction steering angle based on the difference between the actual yaw rate and the required yaw rate of the vehicle, and provide the determined correction steering angle to a steerable road wheel of the vehicle.
However, with the above steering apparatus, the actual steering angle of the steerable road wheel may be increased by the correction steering angle in the direction corresponding to the turning direction of a steering wheel of the vehicle, even when the slip angle of the steerable road wheel has already exceeded a limit slip angle. Consequently, the slip angle of the steerable road wheel increases further, and the lateral force of the steerable road wheel accordingly decreases further, thus deteriorating the turning stability of the vehicle.
To provide suitable steering assistance, Japanese Patent First Publication No. 2000-233762 discloses a steering apparatus for a vehicle, which includes a VGRS mechanism to provide a correction steering angle to a steerable road wheel of the vehicle and an Electric Power Steering (EPS) mechanism to provide assistance power to the steerable road wheel of the vehicle.
In the above steering apparatus, the EPS mechanism is employed mainly for providing assistance power so as to reduce the turning effort of the driver of the vehicle. However, control of the EPS mechanism has not integrated well with that of the VGRS mechanism. Consequently, in some cases, the correction steering angle cannot be reliably provided to the steerable road wheel of the vehicle, thus making it impossible to secure the turning stability of the vehicle.